The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
With reference to FIG. 1, tools used in deviated and horizontal wellbore sections 10, 12 are difficult to move and position in the desired place in the wellbore for downhole operations. Unlike vertical and slightly deviated sections 14, 16, gravity and pipe or coiled tubing strings or wireline assemblies cannot always be used to translate the tool within the wellbore. For example, the string may drag against the walls of the wellbore at some point below where the deviation begins, and there may be a limit on the amount of downward thrust that can be applied to the string from the surface, particularly in the case of coiled tubing. Similarly, in retrieval of the tool, pipe, coiled tubing and wirelines may bind against the inner walls of the wellbore, and together with the weight of the pipe or tubing string or wireline assembly and the tool, the capacity of the string or wireline assembly, e.g., due to the tensile loading that can be withstood and/or the power limits of the retrieval machinery 18 at the surface 20, to pull the tool up may be exceeded.
Thus, crawler mechanisms known as tractors have been used to facilitate translation of downhole tools in deviated wellbores and laterals. Some of these tractors have shortcomings such as, for example, the requirement of an external electrical power supply, complex power and/or drive train arrangements, vulnerability to power and equipment failures, unidirectionality, i.e., the requirement of plural drive assemblies for bidirectional travel, slow insertion and retrieval speeds, abrupt and/or jerky movements of the tool downhole, and so on.
Improvements in downhole tool positioning systems and methods are desired.